TV transmitter control systems

With the continuing downsizing of the technical staff at many stations, the need for simple control of the transmitting equipment becomes even more critical. Staff members are spread out thinner than ever. Now they have to maintain all of the equipment from past years, as well as install new digital equipment for DTV and operate another transmitter. This means that those pieces of equipment that operate with reasonable stability receive less and less attention. Given that a well-maintained transmitter should show such stability, many transmitters receive scant attention during the broadcast day.

Enter the remote control system. Today's crop can be broken into two general types. First, there is the traditional stand-alone remote control system in which a rack-mounted controller is used to monitor a lengthy list of variables while also performing the control function of providing contact closures on demand. The monitored variables normally are represented by a small DC voltage from sampling units in the transmitter. Each measured voltage is then multiplied by a correction factor entered by the operator to calibrate the equipment. At the remote control point, an indication is available of the actual value of each such parameter.

If the equipment is properly calibrated, the operator will have an accurate indication of parameters such as beam current, filament voltage, and forward and reverse power. If the necessary goodies are purchased with the remote control system, the parameters can be shown on a monitor and recorded either to a hard drive or printed out on a paper log. This is the classic type of remote control and should be fairly familiar to all transmitter operators. A large disadvantage of this type of remote control is that it usually requires a really significant number of interface units. The controller itself only looks at a few input voltages for monitoring purposes. At the same time, the control circuits are low voltage/low current contact closures. Placing such contacts directly into the transmitter control circuits is a surefire way to draw arcs and vaporize printed circuit traces.

Expansion modules are purchased for monitoring large numbers of parameters. The sample voltages are wired to the expansion modules, which are switched in and out as needed to make the desired measurements. Happily, switching in the modern units is all electronically controlled by the system controller.

In a similar fashion, those contact closures that are actually provided to the equipment are by relays mounted on external panels. The relays themselves are controlled by the low voltage logic in the controller. These systems have been upgraded with control functions added through improved software.

Now for the second type of system. Most modern television transmitters offer the option of a complete monitoring and control function using software that directly interacts with the transmitter control software. There is no rack full of wires to connect multiple external boxes to the transmitter. One simply plugs in the good old Ethernet plug or a phone line to a modem. The transmitter parameters and controls are then available to anyone with the proper computer software and passwords.

Most of these systems provide remote control of the transmitter itself but do not involve themselves with auxiliary equipment. A small conventional remote control system is often necessary to monitor the STL system, emergency generator, tower lights and building security. Remember, those areas still need attention. The FCC inspector will not be impressed by your ability to adjust the pre-correction circuitry in the exciter remotely if you can't tell if the tower lights are working.

Now, let's move to the real heart of the issue. It can be assumed that the chief engineer does not want to be called every time a storm causes a momentary power break, as long as it can be brought back online simply. In other words, people need to be trained to monitor the remote control readings and control the transmitter.

Today's transmitters are pretty good at protecting the expensive parts if things go wrong. If the operating parameters of an IOT start to go out of acceptable bounds, at least that cabinet will probably be shut down by the transmitter control software. Some accompanying signals should tell the remote control system or the monitor location what the failure mode might be. Often, all this requires is a new application of “ON” to the appropriate control circuit, especially for such problems as momentary power outages. However, this is where training comes into play.

The transmitter doesn't normally know what exactly went wrong, just that a parameter is outside the envelope. But it is not always advisable to keep on trying and trying to bring a system back online after it shuts down. It can be catastrophic in some cases to try again and again to get back on the air.

A prime example of this is a failure in a transmission line or antenna that causes a VSWR trip. If the operator keeps trying again and again to bring the system back online, it is highly likely that the majority, if not all, of the transmission line system may be destroyed.

Finally, an absolute requirement is to regularly check the calibration of the remote control system. It's not hard — just read the meters themselves and compare their indication to that of the remote control system. If this isn't done regularly, the operators slowly but surely lose any idea of exactly how the transmitting system is performing. Remember, it's horrible to get called out of bed to check a problem that turns out to be only a bad calibration.

You can bet on it. It will happen on a dark and stormy night. The telephone will ring and a strange, mechanical-sounding voice will advise you that the transmitter is off the air — or the tower lights are out — or that the power is off and the generator won't start, or some other problem.

Ghosting might not exist in digital systems, but reflections from the antenna are still significant because they can impact the correction process and lead to a higher bit error rate. Control the VSWR levels by adjusting and tuning for the best possible overall response. No standard has been set for VSWR, but the average value should fall well below 1.10.

No area is totally safe from power failures; even some of the most sophisticated transmitter sites are vulnerable. Therefore, standby power plants and power line filtering are highly recommended if not totally necessary.

The least expensive action to increase both equipment life and performance is to perform regular and careful preventative maintenance. It doesn't take long, if done regularly, and the results are beyond argument